The present invention relates generally to a method and composition for improving chemical resistance of a polymeric substrate, and more particularly to a barrier coating composition and method for depositing such coating composition onto a polymeric substrate formed into, for example, a keypad for providing an effective barrier against chemical attacks and other environmental effects.
Elastic polymeric materials are used in a variety of applications where elasticity and spring properties are desired, such as touch keypads, for example, for computers and other devices. Keypads composed of such elastic materials are useful for environmentally sealing a keyboard or data input device to protect internal electronic components from external factors such as weather, moisture, dust, contaminants, and the like. Such keypads typically consist of an injection molded overlay that covers the keyboard circuit card. The overlay contains the individual keys with a conductive contact on the backside to activate the circuit card input. The flexible portion of the key is molded into the keypad and provides the spring constant for the key return. The edges of the keypad are sealed against the keyboard enclosure to produce an environmentally sealed keyboard assembly. Portions of the keypad may be translucent to permit backlighting of the alphanumeric key labels.
An elastic polymeric material useful for such purpose is silicone rubber which possesses excellent resistance to oxidation, ozone, water, and weather. However, solvents including alcohols and the like, corrosive substances, fuels including gasoline, diesel, jet fuel and the like, and nuclear, chemical, and biological warfare decontaminating agents, and the like, are rapidly absorbed into the silicone rubber material causing swelling, physical degradation, and loss of strength and integrity.
Therefore, in light of the above concerns, there is a need to protect elastic polymeric materials including silicone rubber against chemical and other environmental attacks, in order to preserve its physical integrity and extend the life of such materials beyond the typical expectancy and usefulness.
One object of the invention is to impart improved chemical resistance against solvents including alcohols and the like, corrosive substances, fuels including gasoline, diesel fuel, and the like, to elastic polymeric materials such as silicone rubber which have been less than fully satisfactory in resisting chemical attacks, particularly those induced by nuclear, chemical, and biological warfare decontamination agents.
Another object of the invention is to provide a barrier coating composition and method for applying such a composition to a polymeric substrate in a cost efficient and effective manner with no adverse effect on the underlying substrate.
With these and other objects in mind, the present invention relates generally to a barrier coating composition and a method for depositing the barrier coating composition on a polymeric substrate. Preferably, the barrier coating composition advantageously provides significantly improved chemical resistance for the coated substrate. In addition, the coating composition possesses excellent coating adhesion, mechanical strength, and barrier qualities required for long term durability and effective use. The composition and method of the present invention provides low-cost, effective protection of the polymeric substrate. In one embodiment of the invention, the substrate is a silicone rubber keypad.
The barrier coating preferably comprises at least one layer of one or more parylene polymers applied to an outer surface portion of the substrate. The composition is readily applied to the substrate surface portion by first treating the surface of the substrate to remove any contaminants, and depositing at least one layer of a parylene polymer through the process of chemical vapor deposition. The method may be advantageously carried out at room temperature, thus reducing any potential undesirable effects on the polymeric substrate due to heat. In addition, the method yields a highly conformal and continuously uniform coating even on corner portions of the substrate. Optionally, multiple layers of parylene polymers, or one layer thereof, can be annealed for improved barrier performance.
The at least one layer of a parylene polymer comprised in the barrier coating further includes a layer of parylene N, or a layer of parylene C. In a preferred embodiment, a first layer of parylene N is provided, followed by a second layer of parylene C in adjacent contact with the first layer. More preferably, there is further provided a graded interlayer comprised of a transitional mixture of parylene N and parylene C between the first and second layers of parylene N and parylene C, respectively.
In one aspect of the invention, there is provided a method for depositing a barrier coating on a polymeric substrate, which method comprises the steps of treating a surface portion of the polymeric substrate to remove any contaminants; depositing at least one layer of a parylene polymer on the surface portion of the polymeric substrate via chemical vapor deposition; and annealing by heat each layer of the parylene polymer in the presence of a vacuum at an annealing temperature for a sufficient time period.
In another aspect of the invention, there is provided a composition for a barrier coating on a polymeric substrate, which composition comprises a first layer including parylene N on the surface portion of the substrate; and a second layer including parylene C on the first layer of parylene N.
In a particular aspect of the invention, there is provided a method for depositing a barrier coating on a polymeric substrate including a silicone rubber keypad.